Protein characteristics leading aggregation

Many cytosolic proteins are water soluble and their solubility is a function of the ionic strength and pH of the solution. The commonly used salt for this purpose is Ammonium Sulphate, due to its high solubility even at lower temperatures. Proteins in aqueous solutions are heavily hydrated, and with the addition of salt, the water molecules become more attracted to the salt than to the protein due to the higher charge. This competition for hydration is usually more favorable towards the salt, which leads to interaction between the proteins, resulting in aggregation and finally precipitation. The precipitate can then be collected by centrifugation and the protein pellet is re-dissolved in a low salt buffer. Since different proteins have distinct characteristics, it is often the case that they precipitate (or salt out ) at a particular concentration of salt. 

Studies performed over the years have contributed to better understanding of the interactions between proteins and tannins, which are important not only due to their astringency but also because of their impact on food nutritional characteristics, on human health, and on plant metabolism. It is clear that protein-tannin interactions are influenced by several factors, among which polysaccharides could be important because they are also present in tannin-rich vegetables. Much remains to be studied in this field, particularly the specific phenomenon that occurs between proteins, tannins, and polysaccharides that leads to a decrease in aggregation, and further studies are needed involving other salivary proteins and digestive enzymes. 

The lessons we have learned from physics are of a different nature. The history of physics is replete with examples of the elucidation of connections between what seem to be distinct phenomena and the development of a unifying framework, which, in turn, leads to new observable consequences [13]. Indeed, strong evidence suggests that globular proteins share many common characteristics their ability to fold rapidly and reproducibly in order to create a hydrophobic core, the fact that there seem to be a relatively small number (on the order of a few thousand) of distinct modular folds made up of helices and almost planar sheets, the fact that protein folds are flexible and versatile in order to accomplish the dizzying array of functionalities that these proteins perform, and the unfortunate tendency of proteins to aggregate and form amyloids, which are implicated in human diseases. 

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